-United uses the G-pier, usually G3,4 or 5.
-SK flights leave from B, wich means you'll have a long walk to your departure gate, the longest you can have at AMS, as G and B are the 2 most 'outside' piers, (excluding the LCC H-pier) It's about a 20-minute walk at normal pace, not including possible waiting time at immigration, customs and security wich you'll pass when moving into the 'schengen' area.
shower facilities are at the Mercure airside hotel on level 2 of lounge 3, wich is the lounge you'll firest get to when leaving the G-pier. Or if you have access there might be a Star Alliance lounge with showers (SQ??)

Yes, there are showers at the Mercure hotel for the indicated cost.
But you can also take a shower free of charge in the restrooms located opposite Crown Lounge 42. Take your own towel and soap of course.